Are you basing the eating goal on MFP correcting itself with Fitbit sync then?
Meaning you rarely eat the base calories MFP gives, because there is always an adjustment.

If so - those corrections should be based on best info.

You ever measured distance reported by Fitbit compared to known distance - at your avg daily pace (not grocery store shuffle, not exercise pace) - to confirm distance is right?

How long did it take to lose the 38 lbs?
If a lot shorter than 5 months, then the extreme deficit could have caused max adaption by the body - which could include muscle loss if walking was main exercise.

Which means Fitbit estimating higher than reality daily burn, that MFP would be correcting itself with.

During 2 months, how about the more important numbers - measurements, going down or also holding steady?

Attempting to hold to a deficit reasonable for 38 lbs heavier no longer would be - and would be stressful to body to cause it to retain water weight, but spread out.

Or have you reasonable slowed the weight of loss down as the 38 lbs came off?
How much left to lose?

Sorry about all the questions - but they really all provide potential reasons and allow better suggestions.

So #1 is incorrect usage of the tool - but when you have a lot to lose you can have success even then.

But now - not so good because you are causing a greater deficit, which isn't good with little left to lose.

With only 18 to go, the weight loss goal of 1 lb weekly is reasonable, at 10-15 left, 1/2 lb weekly is reasonable.

#2 - that's good for accuracy of daily burn based on activity at least.

#3 - So 7.6 avg monthly, probably more at start, less near end. But that's really the max anyway, so you were well above at the start probably - lost muscle mass I'm betting.
Won't be easy getting that back. Impacts future success at maintaining, or in this case - reaching goal weight.
Yet another reason to set to 1/2 lb weekly to assist not losing anymore muscle.

And to that, follow the eating goal - if 1680 is base calories, that's ONLY on days you are truly sedentary as MFP estimates.
Follow the increased calories from Fitbit.
They may be shown on the exercise diary, but they are not just exercise, or even not that.
But increased daily activity.
Don't make the deficit bigger than it should be.

At this point, 5 months in a diet, or longer actually - probably benefit from coming out of it.
Eating 100 extra daily for a week at a time, another week add another 100, ect.
Keep coming up until you are eating what Fitbit suggests you are burning.
You'll increase water weight and food in stomach weight - but likely lose some stress-retained water weight - likely lose more than gained.

Yes - with minor deficit - body can make better transformations, because you can have better workouts and food is used more for recovery.

That's when you could lose no scale weight, but from all appearances people will think you have lost a lot.

In fact, you start that increase in calories to match what Fitbit is suggesting you burn daily, while you are starting that workout - you'll recover faster and make better progress.

Then at some point you take that 1 lb weekly loss for 3-5 lbs (or whatever to get to 15 left), then switch to 1/2 lb weekly loss.

I'll bet first issue is with increased daily burn - eating enough will be difficult.

Probably - body is easy to adapt to eating lower than it wants and feels is a risk.

So you'll burn less overall, and could reach balance point of not losing anything.